Preheat oven to 350°F and line four 12-cup mini muffin pans with baking cups (recipe makes approximately 40 mini cupcakes). Whisk together boiling water and cocoa, then set it aside to cool completely. Once cooled, whisk in the Bailey's.,Beat sugar, butter and vanilla extract in a large mixer bowl at medium speed until well blended (about 3 minutes). Add eggs and beat well. Set aside.,Whisk together flour, baking soda, baking powder and salt in a medium bowl. Add half of the flour mixture into the sugar mixture and beat until just combined. Add cocoa mixture, then finish by adding the remaining flour mixture.,Fill each baking cup with a slightly rounded tablespoon of batter, to about 1/4 inch below the top of the liner. Bake until a toothpick inserted in the center comes out clean, about 12 to 15 minutes. Cool the cupcakes for 2 minutes in the pan on a wire rack. Holding an edge of each liner, carefully lift the cupcakes from the pan and place them on the wire rack to cool completely.,To make buttercream, beat butter, cinnamon and nutmeg in a large mixer bowl on medium speed until light and fluffy. Reduce speed to low; add powdered sugar and beat until smooth. Add Bailey's and beat on medium speed until airy.,______